Does the audio say 'leVeyt knesset'? Is that right?
Yes, it's correct. ב כ פ preceded by ל change to their soft pronunciation. https://en.m.wikipedia.org/wiki/Begadkefat
It's not necessary to pronounce it as a v but here the audio is being punctilious.
Thank you very much for the explanation and the link!
I learnt at school that we don't say "yom" before Shabbat. "Shabbat is Shabbat". Is it correct?
Depends if you're secular and just referring to the day, then you would say Yom Shabbat. If you refer to Shabbat itself, you wouldn't say Yom Shabbat.
You can say it with or without.
תלוי אל הקידוש
I said "do you walk to a synagogue on Saturday?" and it was marked wrong. I'm reporting it, but is that right?
Why le- and be- instead of la- and ba- ?
1) The definite article would go before כְּנַ֫סֶת
and 2) It is "a synogogue" here.
It's k'nasset? Not k'nesset?
Sorry, typo. Of course it is כְּנֶ֫סֶת. Even with suffix you would have [i], not [a]: כְּנִסְתּוֹ